This invention relates to weather radar and specifically for an airborne weather radar system capable of bright band detection and compensation.
Weather radars are known in the art for detecting and displaying severe weather to a crew in an aircraft on a two-dimensional map display showing range, bearing, and intensity of detected weather patterns such as convective or stratiform weather patterns. Flight hazards due to weather conditions are primarily the result of precipitation and turbulence. Airborne weather radar has been optimized to detect rain since rain drops act as an excellent reflector for the radar's microwave energy. Dry snow and ice crystals are very poor reflectors of radar energy. Wet hail provides the strongest reflection of radar energy. The size of the hail combined with the ability of the bipolar water molecules on its surface to align to reflect radar energy ensures maximum reflectivity levels. Dry hail reflects some radar energy simply due to its size. However, the crystal structure of the dry hail fails to reflect significant amounts of radar energy.
The level of radar returns from an area of weather is used to infer the relative hazard the weather is producing. Encode levels for weather radars are set to infer an aircraft threat from turbulence, lightning, and hail that are associated with convective weather. Some non-convective weather types produce similar return levels that would indicate hazardous conditions if these levels were associated with convective weather. A crew of an aircraft, following a reasonable avoidance strategy based on the assumption of the level of convective weather threat, may avoid regions that are not in fact threatening. This avoidance may result in delayed departures when radar indicates regions of threat while the aircraft is waiting for a clear region in which to plan its departure. Similarly, hold patterns or deviations may be executed during flight when radar-indicated hazardous regions are avoided even when they are indeed non-hazardous.
One such weather radar return is the return from a so-called bright-band. The bright band is a horizontal region in space that produces a larger radar return than the space either above or below it. It is caused by frozen hydrometers falling into air that is above the freezing point of water. The frozen hydrometers may be either hail or snow. As these hydrometers begin to melt, their icy structures are coated with liquid water. Since liquid water has a higher permittivity than ice, the liquid water produces larger radar returns than the original frozen hydrometer. For X-band radars, radar returns are produced by Rayleigh scattering from hydrometers of these sizes. Therefore, small increases in the effective hydrometer size produce large changes in the reflected radar return level. The effective size of the hydrometer may increase during the melting process. With a partially melted snowflake the liquid water coating produces longer liquid water dimensions than those produced by the water drop that results from the complete melting of the snowflake. The initial change from completely frozen to water coated as the hydrometer falls through the warm air produces a substantial increase in the radar reflectivity. The reflectivity then drops as the melting process is completed and a near spherical water droplet is produced.
The large radar return produced by the layer of melting hydrometers is the bright band. While it is likely many situations produce melting hydrometers, generally only non-hazardous stratiform weather produces the large patches of melting hydrometers that produce a bright band on the radar display. Convective weather may also have melting hydrometers but the regions of bright band are mixed with regions of truly higher rain rate induced high radar reflectivity. Therefore detecting and presenting this mixed weather environment as possibly hazardous is appropriate.
The bright band occurs most often in cool spring and fall weather conditions and is observed during approaches and departures with transitions through the freezing level and on the ground with surface temperatures just above freezing. The bright band is associated with stratiform rain and occurs between the freezing altitude and altitudes as much as 3,000 feet below the freezing altitude. In the bright band region, snowflakes and hail begin to melt and are coated with a layer of water producing very strong radar returns. If the radar beam is directed into this region it may cause a substantial portion of the radar display to display a caution or warning due to the fact that the stratiform rain clouds may cover a large geographical region. With red used to indicate warnings and yellow to indicate cautions on weather radar displays, the bright band has been the source of a “red-out” or an “amber-out” condition in which large portions of the weather radar display are indicated as hazardous or cautionary. The bright band produces bright radar reflections with low actual precipitation levels. If the aircraft encounters stratiform rain conditions and red out occurs at or near the freezing altitude, changing the tilt so that the radar beam is either above or below the area of the bright band may improve the radar display. In addition, turning radar system gain below a calibrated position may allow the flight crew to detect any hot spots or areas of severe precipitation that should be avoided. These changes in antenna tilt and radar system gain require crew interaction.
What is needed is a weather radar system capable of automatically detecting a bright band and compensating for its presence or notifying the aircrew of the presence of the bright band.